1. Field of the Invention
The present invention relates to lens control methods using a positional sensor, such as a magnetic resistance (MR) element or an optical sensor, that are applied to focus adjustment in optical systems, as well as lens control apparatuses and cameras provided with the same.
2. Description of Related Art
In cameras with integrated lens, there is a need for miniaturization, and for making it possible to take images at positions as close as possible in front of the lens. Therefore, inner focus type lenses are now mainstream in which the compensation lens and the zoom lens are not coupled mechanically by a cam, but the trajectory over which the compensation lens is to be moved is stored in advance as lens cam data in a controller, the compensation lens is driven in accordance with this lens cam data, and the focus is also adjusted with this compensation lens.
FIG. 9 shows a simple configuration of a lens system of the inner focus type. In FIG. 9, numerical reference 901 denotes a fixed first lens unit, numerical reference 902 denotes a second lens unit for zooming (referred to in the following as “zoom lens”), numerical reference 903 denotes a diaphragm for regulating the light amount. Numerical reference 904 denotes a fixed third lens unit, numerical reference 905 denotes a fourth lens unit (referred to as “focus lens” in the following) that is provided with both a focus adjustment function as well as a function for compensating the shifting of the focus plane due to the zooming (compensation function), and numerical reference 906 denotes an image-pickup plane at an image-pickup element, such as a CCD.
As is well-known in the art, with the lens system configured as shown in FIG. 9, since the focus lens 905 is provided with both a compensation function as well as a focus adjustment function, the position of the focus lens 905 for focusing on the image-pickup plane 906 differs depending on the object distance, even for the same focal length. Continuously plotting the positions of the focus lens 905 for focusing on the image-pickup plane against the focal length for a variety of object distances results in the graph shown in FIG. 10. Zooming without image blur is possible if the cam trajectory corresponding to the object distance is chosen from the plurality of cam trajectories shown in FIG. 10, and the focus lens 905 is moved along this cam trajectory,
In lens systems of the front lens focus type, a focus lens that is independent from the zoom lens is provided, and the zoom lens and the focus lens are coupled via a mechanical cam ring. Consequently, if for example a knob for manual zooming is provided on the cam ring, and the focal length is changed by hand, the cam ring rotates while following this movement, and the zoom lens and the focus lens move along a cam groove of the cam ring, regardless of how fast the knob is moved. Thus, if the focus lens is precisely in focus, then image blur does not occur due to this operation.
On the other hand, in the control for a lens system of the inner focus type, it is common that the data for a plurality of cam trajectories as shown in FIG. 10 are stored in some form (this may be data indicating the cam trajectories themselves or it may be a function of the lens position), the cam trajectory is selected on the basis of the position of the focus lens and the zoom lens, and zooming is performed while following the selected cam trajectory.
FIG. 11 is a graph illustrating an example of a conventional method for tracking the cam trajectory. In FIG. 11, Z0, Z1, Z2, . . . , Z6 denote zoom lens positions, and a0, a1, a2, . . . , a6 and b0, b1, b2, . . . , b6 denote representative cam trajectories that have been stored in advance in the controller. Furthermore, p0, p1, p2, . . . , p6 are cam trajectories that are calculated from these two cam trajectories (i.e. the cam trajectories indicated as a and b). The equation for calculating the cam trajectory is as follows:
 p(n+1)=|p(n)−a(n)|/|b(n)−a(n)|×|b(n+1)−a(n+1)|+a(n+1)  Eq. (1)
According to Equation (1), for example, if the focus lens is at the position p0 in FIG. 11, the ratio at which p0 divides the segment b0−a0 is determined, and p1 is set to the point that divides the segment b1−a1 at the same ratio. The speed at which the focus lens needs to be moved to maintain focus can be obtained from the difference between the focus lens positions, that is, p1−p0, and the time required for moving the zoom lens from Z0 to Z1.
The following is an explanation of the case that the stop positions of the zoom lens are not equal to the boundary positions (Z0 . . . Z6 in FIG. 11) of the data of representative cam trajectories that have been stored in advance. FIG. 12 is a graph illustrating a method for interpolating the zoom lens position, in which a portion of the cam trajectories shown in FIG. 11 has been extracted, and the zoom lens position can be set freely.
In FIG. 12, the vertical axis marks the focus lens position, and the horizontal axis marks the zoom lens position. As for the positions of the representative cam trajectories (focus lens position vs. zoom lens position) stored in the lens controller, when Z0, Z1, . . . , Zk−1, Zk . . . Zn are the zoom lens positions, then the focus lens positions for different object distances are as follows:
 a0, a1, . . . , ak−1, ak, . . . , anb0, b1, . . . , bk−1, bk, . . . , bn
When the zoom lens position is a Zx that is not on the zoom boundaries (Zk−1, Zk) and the focus lens position is Px, then the following Equations (2) and (3) are used to calculate ax and bx:ax=ak−(Zk−Zx)×(ak−ak−1)/(Zk−Zk−1)  Eq. (2)bx=bk−(Zk−Zx)×(bk−bk−1)/(Zk−Zk−1)  Eq. (3)
That is to say, the divisional ratio is determined based on the current zoom lens position and the two zoom boundary positions (Zk and Zk−1 in FIG. 12) that are at the both sides of the current zoom lens position. Then, ax and bx can be determined by dividing internally the position data (ak and ak−1 as well as bk and bk−1) that have the same object distance from the four position data (ak, ak−1, bk and bk−1 in FIG. 12) of the recorded representative cam trajectories with the divisional ratio.
Moreover, pk and pk−1 can be determined by dividing internally the position data (ak and bk as well as ak−1 and bk−1) that have the same focal length (Zk and Zk−1 in FIG. 12) from the four position data (ak, ak−1, bk and bk−1 in FIG. 12) of the recorded representative cam trajectories with the divisional ratio obtained from ax, px and bx, as in Equation (1).
Then, when zooming from the wide side to the tele side, the speed at which the focus lens needs to be moved to maintain focus can be obtained from the difference between the focus position pk serving as the target and the current focus position px, and the time that is necessary for moving the zoom lens from Zx to Zk.
Moreover, when zooming from the tele side to the wide side, the speed at which the focus lens needs to be moved to maintain focus can be obtained from the difference between the focus position pk−1 serving as the target and the current focus position px, and the time that is necessary for moving the zoom lens from Zx to Zk−1. The above-described method for tracking a cam trajectory has been proposed in the related art.
FIG. 13 shows an example of a table of cam trajectory data that is stored in advance in the controller. FIG. 13 shows data A(n, v) for a focus lens position that changes in accordance with the object distance and the zoom lens position. The variable n denotes the change in object distance, and the variable v denotes the change in zoom lens position (focal length). Here, n=0 represents an object distance of infinity, and as n becomes larger, the object distance changes towards close range, and n=m indicates an object distance of 1 cm. On the other hand, v=0 represents the wide end, and as variable v becomes larger, the focal length gets longer, and v=s represents the zoom lens position at the tele end. Consequently, one column of data in the table shown in FIG. 13 traces on a cam trajectory.
The data of the cam trajectories in FIG. 13 are based on optical design values, and are produced as zoom tracking data, but in actual lenses, due to discrepancies in the focal length between individual lens units, the cam trajectories do not necessarily take on the design values. Consequently, in order to track the cam trajectories without image blur, as described above, it is necessary to match the trajectories over which the actual lens moves with the coordinate axis (cam trajectory) of the data in the table.
In actual video cameras, the task is performed of adjusting to which zoom lens position of the stored cam trajectory data the tele end and the wide end correspond.
The following method is known as a focus adjustment method. The difference (focus balance) between the in-focus positions of the focus lens at the tele end and at the wide end is taken as the design value. Then, the zoom lens position is determined such that the shift amount between focus lens position at the tele end and the focus lens position in the middle (intermediate focal length) at which the focus lens is furthest up in a map (FIG. 14 explained below) becomes the design value, and the zoom lens positions at the tele end and the wide end are decided.
This is explained in more detail using FIG. 14. FIG. 14 shows a cam trajectory Sa at a predetermined object distance. Here, the horizontal axis marks the zoom lens position (that is, the focal length), and the vertical axis denotes the focus lens position. With this cam trajectory Sa (for example the cam trajectory for when the object distance is ∞), the difference of the in-focus position of the focus lens at the tele end T and the wide end W is assumed to be zero for the sake of simplicity. Point (1) is taken to be the starting point for the adjustment, and from this point, the focus lens position is reduced in downward direction in the drawing for an amount corresponding to the design value A of the focus balance (position (2)). From this situation, the zoom lens is moved in one direction (to the right in FIG. 14), and takes on the position indicated as (3) when the in-focus position is determined, and this is taken as the zoom lens position Ta at the tele end. The slope of the cam trajectory near the tele end is steep, so that the difference between position (1) and position (3) will be close to the design value.
With the operation up to this point, the focus lens position and the zoom lens position on the actual cam trajectory for which the focal length difference of position (1) and position (2) and the zoom distance difference of position (1) and position (3) fit the design values can be determined. Next, the lens position that fits the designed cam trajectory at the wide end is determined. In this example, the difference of the focus lens positions at the wide end and the tele end is zero, as mentioned above, so that the position (4) at which the zoom lens is similarly moved in the other direction (to the left in FIG. 14), and in-focus is attained becomes the zoom lens position Wa at the wide end. By setting the focus lens position and the zoom lens position at position (4) as the values for the wide end in the table data (FIG. 13) of the cam trajectories stored in the controller, the point of origin for the actual lens trajectories and the ideal trajectories can be matched and zooming without image blur can be achieved.
Ta in FIG. 14 corresponds to the zoom lens position for v=s in the table data (FIG. 13), and Wa corresponds to the zoom lens position for v=0. If the cam trajectory Sa is the trajectory for an object distance of ∞, then position (4) corresponds to A(0, 0) in the table data. With this method for adjusting the cam trajectories, the adjustment is made by fixing the focus lens positions to the design positions and varying the zoom lens positions, so that the stroke of the zoom lens will differ depending on the individual adjusted lenses. Image blur can be prevented by adjusting the displacement amount of the zoom lens for the updated variables v in the table data in FIG. 13.
FIG. 15 shows an algorithm for the case that the focus adjustment is carried out by adjustment software using a controller. The procedure starts at step (abbreviated to “S” below) 1501. At S1502, the position of the zoom lens on the optical axis is set to a start position (position (1) in FIG. 14) corresponding to a position near the peak of the trajectory of the focus lens (cam trajectory).
At S1503, focusing is performed by moving the focus lens with a focus motor. It should be noted that the object distance is set to an adjustment distance (here, it is set to ∞), and an object such as a chart for adjustment is arranged. At S1504, it is checked whether focusing has been achieved, and the focus lens is moved until it is in focus. For the actual focusing, an auto-focus (AF) function is used, and the in-focus position is sought by detecting the position of the focus lens at which the AF evaluation value becomes maximal.
If it has been confirmed at S1504 that focusing has been established, then the focus lens is moved by the design value A for that lens (the position of the focus lens is moved downward in FIG. 14, whereas in actuality it may be moved out toward the object or moved in toward the image plane, depending on the zoom type of the zoom lens). At S1506, the zoom lens is moved from this state toward the tele end T. At the same time, it is judged at S1507 whether an in-focus state has been attained. When the movement of the zoom lens has terminated and an in-focus state has been attained, then the position of the zoom lens corresponds to the zoom lens position (Ta) at the tele end. Then, at S1508, the position of the zoom encoder in that state is stored in Vta as the value that defines the tele end position. At S1509, the focus lens is moved in the optical axis direction for a distance corresponding to the above-mentioned focus balance.
However, if the focus balance is 0 as in FIG. 14, then it is not necessary to move the focus lens. At S1510 and S1511, the zoom lens position (Wa) for the wide end reference is determined by moving the zoom lens to the wide end, similar to the determination of the tele end (Ta) in S1506 and S1507. At S1512, the position of the zoom encoder in the zoom lens position Wa is stored in Vwa as the zoom lens position at the wide end. At S1513, the origin (A00) of the table data in FIG. 13 is rewritten to the focus lens position and the zoom lens position in the in-focus state at the wide end. At S1514, the distance from the origin position to a reference position, such as the reset sensor position or the mechanical limit, is measured, and the focus lens position and the zoom lens position at the reference position are stored as adjustment values. The adjustment is terminated at S1515.
When the power of the camera is turned on, an initialization operation is executed in which the reference position is sought through the above-described process and rewritten to the current lens position (adjustment value) at the time when the lens has been moved to the reference position. With this initialization operation, the positions of the origin of the actual lens trajectory and the table data stored in advance in the controller are matched. Thus, a zooming operation without image blur can be achieved with the method for tracking the cam trajectory described in FIG. 11 while sequentially reading the table data when moving the zoom lens.
In recent years, as image-pickup elements, such as CCDs, become smaller and the number of pixels increases, the need for positional accuracy of the focus lens increases, and cameras have reached the market in which linear motors, such as highly accurate voice coil motors, are used as the actuator for driving the focus lens. In systems using linear motors, MR sensors and magnets that are magnetized at predetermined pitch are used as focus position sensors, and highly accurate position detection is now ordinarily performed by performing an excision process, such as interpolation of output signals of multiple phases that are output from the MR element.
Conventionally, with position detection apparatuses using an MR element, position detection is performed by selecting a phase having a signal component with excellent linearity from sine-shaped signal components with a plurality of phases that are output from the MR element and performing a calculation of interpolating this signal component.
In such position detection methods using an MR element or the like, the detection signal is a combination of a phase component of a sine wave and a wave number component of a sine wave, due to the interpolation process. That is to say, the component corresponding to the wave number is an incremental component, so that it is necessary to settle the origin position by some kind of resetting process. On the other hand, the phase component is an absolute position component that is settled with one cycle of a sine wave.
As described above, in a position detection system combining an absolute position component and a relative position component, if an initialization operation is performed in which the lens is moved to the reference position when the power is turned on, and the current position of the lens is rewritten as an adjustment position, then the relative position component, which corresponds to the wave number component, needs to be reset, but the absolute position component, which corresponds to the phase component, is decided unequivocally, so that it cannot be reset.
Consequently, when gain and offset of the MR element output to be adjusted change due to output variations caused by electrical variations in the signal output of the MR element and changes in the ambient temperature, then the result is that the lens position data that are obtained from the detection signal by an interpolation process fluctuate each time the initialization operation is performed, even though the lens is mechanically at the same position.
This fluctuation amount results in a displacement of the origin in the table data of the cam trajectories stored in advance in a controller or the like, so that errors occur during the tracking of the cam trajectory when actually zooming, resulting in image blur and out of focus.